Radiation detector



Sept' 30, 1952 F|Tz-HuG|| B. MARSHALL ETAL 2,612,610

RADIATION VDETECTOR Filed Nov. 6, 1948 |NvE"NToRs Fitz-Hugh B. Marshall8 John W. Cltmon BY ATTORNEY effects.

` the `phosphor of a narrow Patented Sept. 30, 1952 UNITED STATESPri'rslvr` OFFICE RADIATION DETECTOR Fitz-Hugh B. Marshall, Glenshaw,and J ohn `W.

Coltman, Pittsburgh, Pa., assignors to Westinghouse `ElectricCorporation, `East Pittsburgh, Pa.,.a corporation of Pennsylvania.Application dvembe 6, 194s,seria1.No..5s,'z12' i 10 Claims.

.Our invention relates to ,radiation detection. and more particularlyrelates to detection of high-energy radiation such as alpha, beta, gammaandX-rays.

jIn the prior art devices of which we are aware, considerable`diiiiculties have been encountered in avoiding the dark-currentveffects which are inherent Ain electron multipliers. By dark-currentwemean that current which is present in the`tube when no `radiation isincident on the input phosphor. Attempts have been made to overcomelthisA by obtaining higher concentrations of the electrons from the inputiphosphorphotoelectric surface `combination but the possibilities inthis method are limited.

Itiis accordingly an object of our invention to provide apparatus for`the detection of highenergy radiations with alarge lcurrent output formeasuring weak radiation.

\ "Another object of our invention is to provide apparatus for thedetection of high-energy radiation which produces a minimum ofdark-current ,An ancillary object of our invention is to provide betterapparatus for amplifying the current produced from a radiation detecting`phosphor "s ,a result of small quantities of radiation impinging on thedetecting phosphor.

In accordance with the present invention, we provide a radiationdetector including a storing .phosphor which is capable of storing alarge amount of .energy and which stores this energy over relativelylong periods until stimulated by stimulating radiation and whichreleases this energy rapidly as light when stimulated by the stimulatingradiation. This stimulating radiation is any type of radiant energywhich expedites the release of light from a phosphor. f This may beinfra-.red radiation or radiation of other wave lengths which willexpedite the release of light and which can be separated from ordinarylight by optical filters or comparable means. Heat even though nottransmitted as radiant energy `isan alternative for stimulation, Vand ismeant to be included within the scope of the expression' stimulatingradiation as used herein. The light from the storing phosphor impingeson a photoelectric surface which emits electrons immediately When thelight impinges on it.

A storing phosphor of small area may be used in "certain vsituationsywhere the interception by beam of radiation .is

k suiiicient. Under such circumstances, the light from the storingphosphor may be collected and projected directly on of aphoto-multilplier.

the photoelectric cathode Where a large phosphor isvused, thesensitivity to radiation is increased in proportion to the increasedarea .of the Aphosphoras abroad beam of radiation may `be intercepted."However, ,a problem arises-in the effective `utilization ofthe lightemitted lfrom all elementary Vareas of a broad lbeam radiation detectorsuch as this. In accordance-with 4an in iportant specific embodiment ofour invention, the light 'from the large phosphor islprojected directlyon a large photoelectric layerseparated from the phosphor only by a thintransparent sheet, `and "the resulting electrons emitted by theAphotoelectric layer are concentrated by suitable magnetic or electricfields. l The highenergies given `tothe electrons by the electricfocusing Jfields may `'be employed `for amplification. The `high-energyelectrons may be allowed to `strike one or more electrodes ofamaterialwhich iscapable ofl secondary emission. This would' result inseveral electrons .be-

ing givenl off *for each electron VAwhich strikes an electrode thus-producing an amplification of the, electron "current,

`According to the broader aspects-of our invention, amplification may be4achieved also by allowing theelectrons'accelerated by the focusingfields toiimpingefon an electron phosphor causing the releasevofseveralphotons. These photons may be focused on a photoelecerticlayer where they will cau-se the release of more electrons.

the electron stream emitted by the latter `photo-` electric -layer may-be further "amplified by secondary emission. Thesecondelectron-phosphor photoelectric layer has the advantage, in addition toamplification, ofV allowing one to start again with the'electrons at anegative potential since no energy is lost in carrying photons fromahighly positive toY a `highly negative potential. This enables onetoemploy llower voltages than would otherwisebe required. The novelfeatures that we considercharacteristie .of -our invention are vsetforth with particularity in -theappended claims. The invention itself,however, both as -to its organization and its -methodofoperationtogether with additional objects `and advantages thereof will best beread in` -connection with the accompanying drawing, in `which:

l Figure 1 is va 'schematic showing of a broadbeam vradiation .detector:suitable to embody the principles of .our invention for certainpurposes.

Fig. 2 is -a schematic :showing of a broad-beam radiation'detectoremploying the -secondary emission amplifier ofiFig `l as an `integralpart of the tube instead of externally as shown in Fig. l,v

With the intermediatev e photosensitive surface omitted.

' lati'ng'radiation' 4stimul ating radiation -1sf'contfr'olled so as toth'estoringphosphor p'eriodioally'iora period l used for. certai )this'reective coat isv opaque to the e infrared UVtionzan openingis ofcourse necessary to permit.

entry othe incident radiation or.irifrgt-rede'as ks lioyvfn in Fig. zg;

4I nside the, envelope opposite the storing. plies-1 phor, is` alightesensitive photosurface; I3. This lightsensitive p hotosprface iscapablegof absorbingr-the light photonsernitted by the-storng-phos'- cesiated antimonyfu f .ifA latlng radiation? vviell .cause emissionf-of-lelectronsv electron phosphor and Fig. 3 is a schematic showing of abroad-beam radiation detector Where the focusing is obtained bydeflection of the electrons and amplification is obtained by secondaryeinissiom .l

e Fig. i. 's ka schein @ticshowing of a radiation f detector employing ascreen of small area.

from the light-sensitive phosphor. This filter would be transparent tothe photons remitted by the storing phosphor and opaque to thestimulating radiation.'v

The electron current emitted by the incident radiation le fromthephotoelectriclayer I8 may be. amplied in a photo=rnultiplier tube asshown in Fig. 4. The construction and operation-of the Referring indetail to the drawing, a glassVA e envelope e is coated on the outsideat one end with a storing phosphor 3 sensitive to the radiationto bedetected. This storing phosphor should be capable of storing energy.overa relatively long-,period until stimulated by astimulating rad ationi9 and should be capable of releasing this e energy rapidly as'li'ghtphotons when stimulated Y by a stimulating radiation l G. The energywhich Y. is'stored is that of the detected radiation |2. Y

There are several such phosphors now available commercially such a s,acadmium zinc sulphide or zin'csulphide. Focused onthe storingphosphor isa stiinus. described .be fore.' The which in length is fa. small.fraction ofthe length offene period during' nich no Stirnplatingradi-,V

ation 'ispresent If` a .storing phosphor 4 is .employed ofstantumisulpliidel. activated by cerium and sarna-riuni,` thestimulating radiation should photo-multiplier tube is old in the art andwill thereforeln'ot be described here.

vThe electrons emitted from the photoelectric layerl by thephotons fromthe storing phosphor may also beacceler'ated and focused by electron u.optics of Whichthe principles are well known in vthe art. Thisisaccomplished. preferably' by a series of charged. rings" 22 successivelyat a higher potential' andv with vtheir centers in a straightelectron-sensitive phosphor 24 is preferably ati bei.; of about y10,0 00angstrom's. 'With a cesium v antimony storing phospho r,fth ereY lisfa`highfsensi-kr discMyvith-an aperturein it. ilev the aperture ingradiatonfancl the storing. ,phosphor photons .will beemitted 'by Athestoring phosphor. .L Whilethe solid part ofthe Astimulating radia-..tion andthe Ystoring phosphor, substantially no` photons ,will beemittedby that pl'lospho'r. j Coated.overgthis input'phosphor may beplace'd areflecting surfaceib` capableV ofreflectin'g, the

disc'is `bete/"een the. source 0f light emittedzbythe inputphosphorvvhilebeing transparent to theradiation .,tobe detected. EX-Y arnples ofthis -reiiecting layer, `vvl'iioliQmay be adiation are 'Whitecardboardor thinf aluminum e. c hief requirement for dg that theradiation to be detectedyvill penetratei .-Ifthis refiectingllayer orthe incidentl radia line fromthe center of the input phosphor 3 to thecenterof theelectron-sensitive phosphor 24. Thisjelectron phosphor' 2Emay `be any of lseveral substances such as zinc sulphide phosphor. The

the opposite end of the' tube from the input phosphor 8. .Theelectronsensitive phosphor 24` absorbs` the electrons impinging upon itand emits light photons. .The photons. emitted` by the electron phosphorpass through the glass envelope and are focused byfa .lens on rthephotoelectric' cathode Zf of a photo-multiplier tube.l Theconstruction ofthe photo-multiplier tube iis not claimed in thisinvention and isv shown onlyto demonstrate the' complete process. 'Thephoto- 1 multiplier tube'must of course :be senit'ive to the radiationemittedfby the `electron phosphor V24.

. The electron; `phosphorfphotoelctrlc, `layer combination2.4,`2SbetWeen the focusing Aields anclthe'ampliication fields has ltheadvantage of amplifying. the current by Vproducing several photons foreach of thehigh-energy lectrons iznpinging `cnlthe` eleo'tron phosphor.'Thev larger vnumber of. photonsncause the emission of. the 'largernumber .of electrons from the photoel'ectric layer thusV producinglarger .current than was present duringviocusing.. The electronphosphor- '.-photoeleotric. 'layerlconibination between the focusingi'leldsfand theamplifying .eldsalso hasY the advantage,oisep'aratingthe'twoeld's.

allows one..l to employ a j negative potential `at `,the

' beginningofathe second amplic'z'ation'l stage,

phor 3 and emitting electronsA as al result thereof,;

immediately. An example` of such en -substance is storingphosphor 8.Thisitransparent conducting Vcoatingenablesuseto replace the lelectriccharge lost by-photo'electri emission; l thus maintaining ylightysensitive -photosurface vI8 isa transparent electricallyconductingecoating. 2), Suchas the ina-l rter-ial manufactured'bythePittsburghPlate Glass e Gompany, under; the` trade @name of Nesa, whichltherequired potential lonthephotosensitive surf f storing. phosphor and.theilight-sensitivephosphor 'ateria'ls' areem'ployed such vthatfthe'stimu-"j While employing a highly positive potential at l the 'end ofthe-focusing stage. Otherwise the eld 'nbetween the rst part oftheraniplifying stage vand-thel'ast part of the focusing stage wouldlalmost-ifenotentirely stop the electrons.

ln one. embodiment of our invention; vthe. e1ec-v tron phosphor maybebacked-by a reflecting layer` 28, 'such as a Ythinjco: a .tingl ofvaluminum, which is transparent to the beam of electrons but which `willreflectethelight. photons emitted by the elec.-

tron phosphorfsoas .to cause a greater colleentration of photons `toimpinge on the photo; 'inultiplier tube. f v v The renectinglayer 28 isnotessential and may be omitted if desired. Y By omitting thislayenthe'.` fphotons. going bacli .into ythe interior of. the en-`.velope yand striking they photoelectric layer VI@ would causeiurther.emission of electrons and thus increase thegstrength ofr the pulse'.

Asr shown nFig,-2 itis vnot necessary 'that j Van output; phosphor l beyemployed withan eX- ternal photo-multiplier tube. Instead ele c trodesfifinay beplaoed inside ofv the tube to receive `the' currentof electrons.released from the photo` electriculayer. bye-.the photons from thestoring .phosphor andV amplify the current by secondary emissionemploying `the same principles as `are employed inthe'pl'ioto-multipliertube. The electron beam is `focused onaiirst electrode Where severaladditional4 electrons are emitted. These are then Vattracted'toa-secondelectrodewhere again several additional electrons are lemitted.

`After .continuingithis process with several eleccessively from one toanotherV until they nnally impinge on theanode.

"The potential. of the Afirst of the amplifying electrodesjis preferablyhigher 'than the 4potential of the last of the focusing electrodes.However, in accordance with the vbroader' aspects of our inventionthisiis not necessary. `The lrst of the amplifying electrodes 4may beonly slightly more positive"than the potential of 'the transparentelectrically conducting coating `between thestor- `ingphosphor and thelight-sensitive photoelectric surface" at the .input end of theenvelope.

`Th'isiwillcause theelectrons to slow down after passirrgthe last of thefocusing electrodes but stilliallowthem .toimpinge on the iirstampliiication electrode and cause the emission of some secondaryelectrons although not as many as with `a positive -potential differencebetween the last f focusing .electrode and the .rsti amplificationelectrode;

`jAn alternative method of focusing is shown in Fig."3. Hera` the`electrons Vemitted vby the seni i s itive` phosphor are .attracted .toa nrst row of plates 134. These 45 ldegrees `with the path of (theelectrons and produce a eld which so -orientedas to cause plates are` atan angle of roughly the electrons emittedj from ,a lplate to`.movetoward an anodel32 centrally locatedat theopposite end of' the tubes.This .method .of focusing is employed by "R. C. A. in .their AimageorthicorL An electron impinging on one of these plates will.causetheemissionof several more electrons which Ythen ,proceed to ia.second row of plates where the process is repeated.` This continuesthrough several rows of plates, each row tending to focus the electronscloser together until they nally impinge on the anode. Each successiverow of electrons is positively charged with respect to the precedingrow=so as to causean acceleration of the electrons from the preceding .rowtowardthe succeeding row. i

`lhepradiation to be detected .impinges on the storing phosphor 8 where`it absorbed. v The storing `phosphor stores the'energy of the .incidentradiation until oodedby the stimulating radi-ation l0. When flooded, bythestimulating radiation lll, the storing phosphor emits photons `which,pass through the glass .envelope and `impinge `on the photoelectric,layerf lThe photoelectric layer `absorbs the photonsfand emitselectronsas a resultlthereof. These electrons are accelerated by the fieldproduced by the transparent conducting layer 4and the successive :ringelectrodesJZ. -These rings `are sooriented and apote-ntial iis soapplied`to them as to `cause the electrons to tend to come to a focus. Thestream of electrons thus focused is caused to impinge upon theoutputvphosphor 24. The output phosphor absorbs the electrons and givesoi light immediately. The light may then be focused on the cathode 26 ofa photo-multiplier tube for further amplification. In thephoto-multiplier tube,

electronsI are emitted from the cathode and are successively4accelerated from one electrode-to another, each time gainingadditional' electrons lin the group' due Vto secondary emission until`they finally impinge upon an anode. By the time the electrons :impingeupon the anode, the current hasbeen greatly ampliiied.

By. employing the storing phosphor andthe stimulating radiation, .wecause the energyof the radiation'to bedetected, which has been collectedby the storing :phosphor over a comparatively long :period to bereleased and amplied in one pulse which may be of short duration. Theduration `preferably is 1/100 of the time interval during which :theenergy cf the radiation to be detected is being stored. The dark-currentnoise-Which is present during this short interval is the characteristicdark current and is small in proportion tothe strength of the signalpulse current. `At the `same time, we have achieved considerableamplification by releasing a large number of electrons at one time,`thus starting with a stronger pulse to be amplied.

`While there .are `very definite advantages, as described above, toemploying the storing phosphor ,8 and the vstimulating radiation l0 inconjunction with the methods of amplicationdescribed herein, thesemethods of amplication maybe employed without use of the storingphosphor. This'mayfbe achieved by employing an 'input `phosphorsensitive to the radiation to be detected and `capable of emittingphotons immediately as a result thereof and a photoelectric phosphorcapable of absorbing the photons emitted .by the input phosphor andemittingv electrons immediately as a result thereof. Examples of such acombination of input `phosphor'and photoelectric phosphor are given inapplication Serial No. 752,942, ledJune 6, 1947. The `elec-- tronsemitted bythe photoelectric phosphor are then accelerated". `and`focused by electron optics as shown in Figs. '1 and 2.01' by secondaryemission as. shown in `'l.-"igs..2 and 3.

:Since .numerous Achanges maybe `made in the above-describedconstruction, and different embodiments-of .the invention maybe madewithout departing from 'the spiritand scope thereof, it is intended that:all matter contained in the 'foregoing description or shown in theaccompanying drawing shall be interpreted as 'illustrative and not .inralimitingsense.

fwe claim asour invention:

l. In combination, an :input `phosphor capable of storing a portion 0fthe energy ofradiation impinging thereon, 4a photoelectric surface, -anelectron-optical .focusing system, lan electron phosphor, a `source ofstimulating radiation to which said input phosphor isisensitive toaccelerate the 4release ofysaid-stored energy'direc'ted toward .saidinput phosphor, means for changing theintensity `of said stimulatingAradiation at predeterminedintervals. i i "2. Incombination, a screencapable of absorbing the radiation to vbe detected and capable :ofemitting electrons as a result thereof when a stimulating radiation-isincident thereon; a source of said stimulating vradiation, electricandmagnetic fields capable of Afocusing the electrons emitted by saidscreen onto a small area, means light and causing emission-of electronsin com'-, paratively large numbers onlywhen a stimulating radiation isincident on it, a source of said stimulating radiation directed Vat saidinput phosphor,

anpelectron-optical focusing system capable of phosphor, aphoto-multiplier. ,y ,y

lL-A1 radiation `detector comprising an' .input screen'sensitive to theradiation tobe vdetected and capable of emitting electrons 4as ,aresult' thereof, said screen being capable, ofQemitting electrons incomparatively large numbers only whenv a stimulating radiationisincidentthereon,

a source of said stimulatingradiation .incident onf` said screen, anelectron-optical focusing eld ca-- pable `of focusing-said electrons onone or more focusing saidv electrons emitted by'V said input'` phosphorontov an output phosphor, said output electrodes, a plurality ofelectrodespa eld between.V said electrodes capable'of causingtheelectrons emitted by one of said electrodes to be ae-` celeratedtov/ard other of said electrodes. Y y

5. A radiation detector comprising an input phosphor sensitive to theradiation to be detected andcapable of emitting electrons as 'a resultythereof, said phosphor being capable ofernitting electrons incomparatively large numbers only when a stimulating radiation isincident thereon,` l

a source of said stimulating'radiation incident on said phosphor,anrelectron-optical focusing` eld capable oi' focusing said electrons onone or more electrodes, a plurality of said electrodes, a eld betweensaid electrodes: capable of causing the electrons emitted by one of saidelectrodes` to be accelerated toward other :of said electrodes, in-vstruments connected tosaid electrodes to record the currentbetween them.

6, A radiation detector-.comprising an input' phosphor sensitive to theradiationitojfbe cletected and Capable ofY emittingr` electronsy as aresult thereof, fsaid phosphor :being capable .of emitting electronsincomparatively large numbers only When a stimulating'radiation isincident thereon, Aa source of said stimulating radiation incident onsaid phosphor, saidA stimulating radiation beingfpresent at periodicintervals',-an,jelec .tron-optical focusing field capableof focusing:

f saidl photoelectric layer, andmeans `for changing the intensity ofsaids'timulating radiation atY said electrons on oneor moreelectrodes, aplu-r rality of electrodes capable of a high degree of` secondaryemissionon being lbombarded by 'elec'v trons, aneld between saidelectrodescapable of causing the electrons emitted by one of said elec-ytradesy to be accelerated toward other of saidA electrodes.

1. Avradiation detector comprising an input phosphor sensitive to theradiationV to be detected'and capable of emitting Velectrons asV a refvsultjthereoi, said phosphor being capable of emitting electrons v,incomparatively large numbers only when a stimulating radiation` isincidentthereon,` a source,l of said stimulating radiation incident onsaid phosphor, said stimulating radiation J being present Vat periodic"intervals, a plurality of electrodes capable of a high degreek ofLsecondary emission on being bombarded by electrons, a held between saidelectrodes capable r of causing the electrons emittedrbyone of saidNumberv Y y electrodes to beraccelerated toward otherof saidelectrodes., said eldcausing said ,electrons to tend tofocus intoanarrow beam. 8. A radiation-detector comprising an inputphosphor-sensitive tothe radiation to be del tectedandfcapable ofemitting electrons as a result thereof, said input phosphor ybeingcapable of emitting electrons iny comparatively large ynumbers onlywheny a stimulating radiationis incident thereorr a` source" ofzsaidstimulating radiation incident on said input phosphor, anelectron-,optical focusing eld capable of vfocusing said electrons onanelectron sensitivephosphor,

said electron'sensitive phosphor, a,photomulti plier'capable ofabsorbing thelight emitted by said electron Vsensitive phosphor andproducing a current proportional to the vintensityfof the light emittedby said electron sensitive phosphor` j 9; In comblnatiomafstoringphosphor capable Vof absorbing; radiation to be detected, of storing theenergy derived from that radiation over a shortperiod of time and` ofemitting that energy as 'lightf-more rapidly in response Ytoinfra-'redradiation incident thereon,N apparatusqfor Ycausing: saidinfra-redradiation to imping'e on, said phosphor periodically, aphotoelectricVla'yerso located 'with respect to said phosphor that theA lightproducedVbyy said phosphor -Will yiin'pinge on said photoelectric layer, vandmeans for acyby vsaid photoelectric layer.- 4

, ,10. Alradiatior'i` detector comprising a storing phosphor sensitivetoa stimulating radiation, a sourceof said stimulating radiation, aphotoeleccelerating and focusing the electrons produced` tric layercapable of'absorbing the light emitted by said storing'phosphorandemittingelectrons as a result thereof, vsaid photoelectric layerbeing so locatedwithilrespect to said Vstoring phosphor thatradiationemitted `by said storing phosphor will impingelon saidphotoelectrc layer, means for amplifyingfthe electron'currentj emittedby predetermined` intervals.

1 v y ',Frrz-HUGH BMARSHALL. y j JOHN W. coLTMAN.

, *,nEFnnnNcEs crrsn l The following references are of record in theille .ofy this i patent:

UNITED srATEs PATENTS `"Name Date 1,648,058 Parker Nov. 8, 19271,996,492 Schrot'er, Apr. 2, 1935 2,158,853 f Coolidge May 16, 19392,198,479 Langmuir Apr. 23, k1940 2,227,018.' I Schlesinger'- Dec. 31,'1940 2,239,887; Ferrant 'Apr. 29, 1941 v 2,270,373 Kallmann et al, Jan.20, 1942 Y 2,395,099, Cage Fed 19, 194el 2,407,564, Martin et al Sept..10, 1946 2,459,778 I`larson. y Jan. 18, 1949 f OTHER 'REFERENCES'Microsecond Measurement of the Phosphorescence of X-ray'FluorescentScreens, Journal of Applied Physics,'vol.- 18, Junef1947, pgs. 512-518.

